This invention relates to processes for manufacturing monolithic integrated circuits which include overlaid layers of dielectric materials, in particular layers of silicon oxynitride and silicon nitride.
The adjacency of dielectric material layers having the same or different compositions is a common occurrence in today's technology at several manufacturing stages of integrated circuits. In such processes, a second layer is subsequently formed over a first layer of dielectric material. That second layer may overlap the first at a next stage so as to completely cover it, as is the case with certain passivating steps, for example. However, before forming the second layer, operations are often carried out such as the formation of a sacrificial layer which is then etched away, either fully or partially, or such as the partial removal of the first layer itself without using a sacrificial layer. In any case, the second dielectric layer will have bottom surface portions in direct contact with top surface portions of the first.
Dielectric material layers usually function to provide electric insulation between conductive layers and to protect underlying structures of the integrated circuit against contaminants (impurities, moisture) or impacts. Additionally, the provision of successive layers ensures protection for the device even when any one of the layers becomes damaged, e.g., by the formation of fine cracks. Therefore it is important that no regions be allowed to have less than perfect adhesion between the overlaid layers in the areas of contact. Nonetheless, some materials develop peculiar adhesion problems at the interface which arc not fully understood by those skilled in the art.
These problems already exist on account of certain inherent properties of the materials. In fact, when these are placed in layer form over another material, they develop by inherent stresses a more or less marked tendency to deform the underlying structure into a more or less curved shape, which may be concave or convex, according to whether the stress is a tensile or a compressive one. Where the adjacent underlying structure is also formed of another layer of dielectric material, the magnitude of the force acting on the two layers may, even when the stress is of the same type and perhaps of equal value, be different due to fact that the thicknesses are generally different. At high values of that stress, the layers tend to separate and possibly "delaminate". In general, spontaneous delamination occurs where the stress at the interface exceeds the molecular adhesion forces acting between the two layers.
This problem is aggravated by the application of external mechanical loads which add further stress, e.g., while dicing the wafer on which the circuit is formed. Other significant influences may include changes in temperature during the circuit manufacturing cycle, and environmental chemical attacks; these effects are apt to degrade the interlayer bonds.
The difficulty of achieving adhesion of the layers is further enhanced by certain methods, which are commonly applied during the manufacture of an integrated circuit.
In the first place, current deposition techniques generate layers with uneven regions especially near interfaces. As those skilled in the art are well aware, the dielectric material layers are customarily formed (excepting the first, which may be grown thermally) by deposition within a reactor, using a chemical vapor deposition (CVD) technique. The chemical precursors of the elements to be deposited are reacted while in a gaseous state. The chemico-physical properties of the layer and the reaction rate are controlled by process parameters such as pressure and temperature, and by the possible ignition of plasma induced by an RF drive voltage. However, a dielectric deposited by a CVD technique is never truly uniform throughout its thickness: the first tens of nanometers have different composition characteristics from the remainder of the layer because the surface electron states of the underlying layer, where the reaction takes place, affect its properties. Especially where the plasma-assisted CVD technique is used, from plasma ignition to the moment that the reactor enters a steady state of operation, a short settling and stabilizing period must be expected before the deposition rate itself attains steady state and becomes uniform. This may introduce increased uneveness in the characteristics of the deposited layer.
On the other hand, it is possible for an interruption in the deposition of a layer to cause saturation of the surface bonds, making the adhesion of the next layer more difficult still. In addition, where the layer on which the deposition is carried out has been subjected to a previous processing step, the surface to be deposited will in general be altered. For example, in the event of a previous plasma etching step by ion bombardment (a process used for removing a sacrificial layer, for example), damage is inflicted to the surface under the mechanical and physical aspects, possibly resulting in a change of density. Further, a surface alteration under the chemical aspect may occur due to the incorporation of residual spurious chemical elements from an etching process (etching chemical elements or residues from an attached layer).
The problem is of special importance because the trend in the art is nowadays toward assembling integrated circuits into inexpensive plastics packages. A characteristic feature of plastics materials is that they are pervious and, therefore, provide no barrier against moisture, while in addition, the circuit encapsulating operation--including resin heating and subsequent cooling, with attendant release of moisture from the resin--may easily admit water into the package. This water can eventually lead to corrosion of the conductive layers, or in any event, make the integrated circuit unstable, impairing its performance.
Thus, the technology for making such devices provides for the use of silicon oxynitrides and nitrides as dielectrics. These materials do provide a barrier against migrating water, and they can also effectively prevent contamination from sodium.
On the other hand, overlaid silicon nitride and oxynitride layers--whether in the nitride/nitride or oxynitride/oxynitride or nitride/oxynitride combination--have a marked tendency to separate after deposition, because they are highly rigid materials inherently likely to build up internal stresses.
In order to improve adhesion between dielectric materials, some techniques have been developed which provide treatments of the surface of an underlying layer prior to forming the next.
One prior solution consists of bombarding the surface of a layer to be deposited with ions of an inert gas, such as argon or nitrogen, under plasma (sputtering process). In this way, the roughness of the surface is enhanced mechanically to provide increased gripping area for the reactants to be deposited, which improves adhesion. That technique applies to the instance of oxide layers deposited using tetraethylorthosilicate as the precursor (known as TEOS).
Another solution which can be adopted where the surface of the underlying layer includes spurious chemical elements is the provision of a cleaning step by plasma etching, using an oxygen or mixed N.sub.2 and NH.sub.3 plasma, or by a wet etch. In this way, it becomes possible to provide improved chemical uniformity at the interface.
All these prior approaches may also be used in succession or simultaneously for improved efficiency, and have proved adequate with certain materials. However, particularly with silicon oxynitride and silicon nitride layers, they are ineffective to prevent separation of two successive layers, which not infrequently occurs some time after deposition, as can be verified experimentally.
The present invention advantageously provides a method for causing improved adhesion at the interface between layers of dielectric materials during the manufacture of integrated circuits having multilayered structures. Specifically, the method is also effective with layers of silicon nitride and oxynitride in the nitride/nitride or oxynitride/oxynitride or nitride/oxynitride combination, thereby improving the reliability of integrated circuit devices encapsulated in plastics packages.